The invention relates to a process for cleaning and cooling partial oxidation gases containing dust-like impurities.
It is generally known that partial oxidation gases can be obtained by the gasification of solid and/or liquid fuels such as coal, coal dust or heavy heating oils by concurrent flow gasification of the fuel and air or oxygen. The partial oxidation gases then contain essentially only solids which are of dust-like structure. The gases, on the other hand, do no longer contain any amounts or any mentionable amounts of higher boiling hydrocarbons such as oils, tars, benzene, phenols or similar compounds. A wellknown and broadly used process for the concurrent gasification is the Koppers-Totzek process which is being used at normal or only slightly increased gasification pressure.
The dust-like impurities of these partial oxidation gases consist essentially of soot, coal dust and fly ash (flue dust) which occur as gasification residue in amounts of about 60 to 100 g/Nm.sup.3. These are entrained with the gases out of the gasifier and must therefore be eliminated from the gas flow. The partial oxidation gases also contain gaseous impurities, in particular H.sub.2 S, HCN, COS and CO.sub.2 which likewise must be eliminated in the course of the further processing.
In view of the absence of the mentioned higher boiling hydrocarbons in these gases it has been the practice to pass the hot gases received from the gasifier into socalled scrubbers (cooling washers). The gases in these scrubbers were cooled by direct contact with water and the dust-like impurities were thus washed out. A diagram of this type of process has for instance been published in the publication "Erdol and Kohle," Vol. 28 (1975), page 84, FIG. 6.
This procedure, however, has the disadvantage that it results in comparatively large amounts of waste water since the wash water drained from the scrubbers because of the dust-like impurities suspended therein cannot be discharged to the outside without preceding purification. Besides, the wash water depending on the pressure employed in the cooling and washing step contains more or less large amounts of offensive smelling and poisonous components such as H.sub.2 S, HCN and COS in solutions which likewise must be eliminated.
The treatment of the waste water occurring in the scrubbing of partial oxidation gases therefore constitutes presently a serious problem because of the constantly increasing governmental restrictions and requirements for protection of the environment. These problems are accentuated by the fact that the coal gasification in accord with other branches of modern technology tends to aim at increased throughputs which again result in increased waste-water amounts.
Because of all these problems one could think of eliminating the dust-like impurities from the gas by a dry gas purification. It has, however, been found that, with the prior art dust separators, a complete elimination of the dust-like impurities from the gas stream is impossible because the impurities always include a proportion of particles below 10 micron grain size which are not taken up by the dust separators. The use of electrofilters or similar devices is likewise not advisable. The reason is that in case of any interference in the operation of the gasifier unreacted oxygen may reach the electrofilter together with the partial oxidation gas and may cause an explosion.
It is therefore an object of the present invention to provide for a process of purifying and cooling partial oxidation gases which contain dust-like impurities which process on one hand should assure, if possible, a complete elimination of the dust-like components and, on the other hand, should result in only a relatively small waste-water amount. In addition, the process must be absolutely safe and must have a high flexibility in regard to the type of gas cooling.